1. Field of the Invention
The present invention relates to a ink ejection head unit for ejecting ink drops and an image forming apparatus, such as a printer, a facsimile, a copier, a plotter, a multiple function machine, etc., having the ink ejection head unit.
2. Discussion of the Background Art
An image forming apparatuses using a liquid ejection printing system, ink jet printing apparatuses that eject ink drops and form an image on a sheet conveyed are well known. Such image forming apparatuses are of two types: A serial type, in which a printing head ejects ink drops while moving in a main scanning direction, and a line type, in which a printing head ejects ink drops without moving in the main scanning direction.
In either case, an ink supplying system included in an ink jet printing apparatus typically includes a main tank and a sub tank in a printing head and an apparatus body, respectively. Specifically, ink is replenished from the main to sub tanks, and the sub tank supplies the ink to the printing head.
In such an ink supply system, there are provided a supply tube for supplying the ink from the main to sub tanks and a flexible film-like member or membrane serving as a damper for keeping a constant pressure in the sub tank. However, into those devices air gradually penetrates as time elapses, eventually accumulating in the sub tank. Further, when the main tank is attached or detached, a small amount of air also enters the system and is ultimately supplied to the sub tank together with the ink.
To counteract the above-mentioned tendencies, Japanese Patent Application Laid Open No. 2002-86748, a conventional sub tank that includes an ink chamber that deforms in accordance with an ink amount stored therein while maintaining negative pressure. The ink chamber includes separate ink introduction and air evacuation sections at its upper section and an ink supply section at its lower section. The ink introduction section is elastic and includes a replenishment valve system having a valve seat having an ink introduction path, a valve body, and an elastic member for pressing the valve body against the valve seat so as to shut off the ink introduction path. In the evacuation section, there is provided an elastic sealing system having a closed slit at its center.
Thus, the sub tank can suppress accumulation of the air therein by separately providing the ink induction path from the air evacuation section and evacuating the air therefrom.
However, since the slit is made of elastic material, the sub tank does not contribute to improving air tightness of the air evacuation section.
As a result, when the ink chamber enters a negative pressure state, air tends to slip in through the slit.
Further, Japanese Patent Application Laid Open No. 2005-014342 discloses a system that includes an atmosphere open valve for communicating a container of a sub tank with atmosphere.
However, if foreign matter invades and is attracted thereto during either assembly or usage of the valve body and the valve seat of the atmosphere open valve system, a gap appears and an air-tight state cannot be maintained. As a result, air invades the sub tank through the atmosphere open mechanism.